Fix Body#extract_parts leaving boundary's CRLF at head of raw_source

[hikmetba-bit]

Summary

Fixes #1171.

RFC 2046's multipart delimiter is "dash-boundary transport-padding CRLF" — the CRLF that terminates a boundary line belongs to the delimiter itself, not to the part content that follows it. Body#extract_parts's regex only looks ahead at that trailing CRLF (via (?=\s*$)) without consuming it, so it gets left dangling at the head of the next part's raw_source instead of that part's actual content starting there:

m = Mail.read('spec/fixtures/emails/multipart_report_emails/multipart_report_multiple_status.eml')
m.body.parts[0].raw_source[0, 40]
# => "\r\nContent-Type: message/delivery-status" (before)
# => "Content-Type: message/delivery-status;" (after)

This was invisible for most users because Mail::Part#initialize lstrips whitespace/CRLF from its input, so the parsed body ends up correct either way. But raw_source itself is wrong, which matters for anyone who reads raw_source directly (my case, and the original reporter's).

Fix

Strip that one delimiter-owned line break from each part's content after the first (the preamble at index 0 is unaffected, since nothing precedes it). Kept the existing lookahead-based split and blank-part/missing-closing-boundary logic untouched — just clean up the resulting part strings afterward, rather than restructuring the regex (which the existing final_separator == "--#{boundary}--" exact-match check depends on).

Testing

• Added a regression test in spec/mail/body_spec.rb asserting each part's raw_source starts at its actual content, not a leading \r\n.
• Verified the new test fails against the code without this fix (diff shows the exact dangling \r\n), and passes with it.
• Reproduced the original issue's exact fixture (multipart_report_multiple_status.eml) and confirmed raw_source is now clean for all parts.
• Full suite: bundle exec rspec spec/ — 1837 examples, 0 failures, 4 pending (all 4 pending pre-exist on unmodified master too, unrelated to this change — address-list/date/sender-field edge cases).

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